In several recent studies and on-going developments for advanced rotorcraft, the need for variable or multi-speed capable rotors has been raised. A speed change of up to 50% has been proposed for future rotorcraft to improve overall vehicle performance. Accomplishing rotor speed changes during operation requires both a rotor that can perform effectively over the operational speed-load range, and a propulsion system that can enable these speed changes.
Rotorcraft propulsion is a critical element of the overall rotorcraft. Unlike fixed wing aircraft, the rotor propulsion system provides lift and control as well as forward thrust. As a result, the rotorcraft engine-gearbox system must be highly reliable and efficient. In addition, the gearbox system must be kept at minimum weight. Presently, the propulsion system accounts for up to 25% of empty vehicle weight. The drive system accounts for up to 72% of the total propulsion system weight. Future rotorcraft trends call for more versatile, efficient, and powerful aircraft, all of which challenge state-of-the-art propulsion system technologies. Variable speed rotors have been identified as having a large impact on many critical rotorcraft issues.
Currently, rotor speed can only be varied a small percentage by adjusting the speed of the engine. The variation in rotor speed is generally limited by engine efficiency and stall margin, permitting speed changes limited to approximately 15% when used in current tilt-rotor applications.
There is a need for a transmission with a high-range ratio (1:1) for hover mode operation and low-range reduction ratio, such as for example 50% (2:1), through a speed change mechanism, for cruise mode operation. A transmission of this type could be incorporated as an element within the overall propulsion system resulting in overall ratios of 50:1 to 100:1 in the aircraft.
It is commonly recognized that variable speed propulsion is required for the design of future advanced rotorcraft. Reductions in rotor speed are required to limit the advancing rotor tip speed and reduce rotor noise.